1. Field of the Invention
The present invention relates to a process for preparing alcohols by hydroformylation of olefins or olefin mixtures in the presence of a cobalt catalyst, removal of the catalyst and subsequent hydrogenation. The invention further relates to alcohols prepared by the process, and mixtures obtained by the process.
2. Discussion of the Related Art
Higher alcohols, in particular those having from 7 to 25 carbon atoms, may be prepared by catalytic hydroformylation (also known as the oxo process) of the olefins having from 6 to 24 carbon atoms to form aldehydes having from 7 to 25 carbon atoms and subsequent hydrogenation of the aldehydes. The alcohols can be used for many purposes including as solvents or as precursors for detergents or plasticizers.
A large number of processes for the hydroformylation of olefins are known in the literature. U.S. Pat. No. 5,268,514, U.S. Pat. No. 536,912, U.S. Pat. No. 5,462,986 and U.S. Pat. No. 5,463,147 describe the hydroformylation of 1- and 2 butene-containing mixtures, in which the 1-butene is reacted in a heterogeneous reaction, i.e. in a multiphase system, if appropriate with addition of a phase transfer reagent or solubilizer, in the first stage and a homogeneously dissolved catalyst is used in the second stage. In U.S. Pat. No. 5,268,514 and U.S. Pat. No. 536,912, rhodium catalysts are used in both stages, while according to U.S. Pat. No. 5,462,986 and U.S. Pat. No. 5,463,147 rhodium catalysts are used in the first stage and cobalt catalysts are used in the second stage. According to U.S. Pat. No. 5,268,514 and U.S. Pat. No. 536,912, the olefin which is not reacted in the first stage, predominantly 2-butene, is hydroformylated in a homogeneous phase and in the presence of rhodium as catalyst in a second stage. In U.S. Pat. No. 5,462,986 and U.S. Pat. No. 5,463,147, this procedure is defined more precisely in that the olefins which have not reacted in the first stage leave the reactor in gaseous form together with carbon monoxide, hydrogen and butene formed by hydrogenation, i.e. an intermediate separation of the olefins is carried out. The gas which has been separated off is, if appropriate after compression, passed to the second hydroformylation stage.
GB 1 387 657 describes a two-stage hydroformylation in which the reaction product of the first stage is discharged in gaseous form and, after the aldehydes or alcohols have been condensed out, the offgas from the first stage, which comprises unreacted olefins, is partly recycled to the first stage and the other part is passed to a second reactor.
A further variant of a two-stage hydroformylation is described in U.S. Pat. No. 4,447,661. Olefins are hydroformylated to conversions of from 50 to 90% in a first stage using a cobalt catalyst, the cobalt catalyst is separated off from the reaction mixture and the aldehydes formed are introduced together with the unreacted olefins into a second hydroformylation stage. The ligand-modified cobalt catalyst used here brings about not only the hydroformylation of the olefins, but at the same time hydrogenation of the aldehydes to the alcohols.
U.S. Pat. No. 6,482,992 describes a process for the multistage cobalt- or rhodium-catalyzed hydroformylation of olefins having from 6 to 24 carbon atoms to produce alcohols and/or aldehydes, in which the olefins                a) are hydroformylated to a conversion of from 20 to 98% in a hydroformylation step,        b) the catalyst is removed from the liquid reactor output,        c) the liquid hydroformylation mixture obtained in this way is separated into a low-boiling fraction comprising olefins and paraffins and a bottom fraction comprising aldehydes and/or alcohols, the olefins present in the low-boiling fraction are reacted in further process stages comprising the process steps a, b and c, and the bottom fractions of the process steps c) of all process stages are combined.        
This process is preferably carried out so that the liquid reactor output from the hydroformylation step a) is a homogeneous liquid phase. The cobalt or rhodium catalysts are preferably used in such a way that they are homogeneously dissolved in the liquid reactor output from the hydroformylation step a).
U.S. Pat. No. 6,331,657 describes a process for preparing higher oxo alcohols from mixtures of isomeric olefins having from 5 to 24 carbon atoms by means of a two-stage hydroformylation in the presence of a cobalt or rhodium catalyst at elevated temperature and superatmospheric pressure, in which the reaction mixture from the first hydroformylation stage is selectively hydrogenated, the hydrogenation mixture is separated in a distillation into crude alcohol and low boilers comprising predominantly olefins, these are fed to the second hydroformylation stage, the reaction mixture from the second hydroformylation stage is once again selectively hydrogenated, the hydrogenation mixture is separated in a distillation into crude alcohol and low boilers, the crude alcohol is worked up by distillation to give pure alcohol and at least part of the low boilers is taken off to discharge saturated hydrocarbons from the system.
The residual amounts of cobalt catalyst remaining in the organic phases obtained after the hydroformylation in the presence of a cobalt catalyst are generally less than 5 ppm of cobalt (calculated as metal). Even these small residual amounts of cobalt can with increasing period of operation have adverse effects both on the hydrogenation and on the work-up by distillation.
The hydrogenation catalysts may be deactivated by the residual cobalt in the organic phase as the period of operation increases. In particular, cobalt deposits on the catalyst surface are observed on prolonged operation.
Apart from the deactivation of the catalyst, the cobalt deposits also have an adverse effect on the hydrodynamics and the mass transfer and/or heat transport in the hydrogenation reactor.
U.S. Pat. No. 6,365,783 discloses a two-stage process for preparing alcohols from olefins or olefin mixtures. In this process, the feed olefin is hydroformylated to an extent of from 50 to 90% in the presence of a cobalt catalyst in the first reaction stage. After the catalyst has been separated off, the unreacted olefins are separated off by distillation from the reaction output and the olefins which have been separated off are reacted in a second hydroformylation reactor. The hydroformylation products from both stages can be hydrogenated to form the corresponding alcohols. In both reaction stages, the catalyst used is Co2(CO)8 or HCo(CO)4 which is produced outside the hydroformylation reactors. The cobalt catalyst is removed from the reaction mixture from the hydroformylation by extraction with a base prior to further processing.
In most of the cobalt-catalyzed hydroformylation processes known from the literature, the cobalt catalyst (e.g., HCo(CO)4 or Co2(CO)8) is destroyed by oxidation after the hydroformylation step. This is generally achieved by reacting the output from the hydroformylation with air in the presence of an aqueous phase, with the cobalt(II) salts produced in this way being extracted into the aqueous phase. The aqueous phase is separated off, for example, by decantation in a phase separation vessel or in other apparatuses suitable for this purpose. After the organic phase has been separated off from the aqueous phase, it is passed to a catalytic hydrogenation.
In DE 102 27 995.0, the concentration of cobalt compounds (calculated as metallic cobalt) in the hydroformylation mixtures is reduced to values below 0.5 ppm by mass by extraction with water. The operating life of the hydrogenation catalyst in respect of satisfactory hydrogenation performance can in this way be increased to about 2–3 years. However, a disadvantage which remains is that the first catalyst layer in the hydrogenation of the extracted hydroformylation mixture becomes conglutinated by deposited metallic cobalt and possibly other substances. As a result, an increasing differential pressure builds up in the reactor and the flow of the liquid to be hydrogenated becomes nonuniform in the subsequent catalyst layers, so that the hydrogenation performance decreases. The hydrogenation has to be shut down at intervals for the first catalyst layer to be loosened or be replaced by fresh catalyst.